Alkene separation process

ABSTRACT

A process for the oxidation of a C 2  to C 4  alkane to produce the corresponding alkene and carboxylic acid which process comprises separation of the alkene from a mixture of the alkene, the alkane and oxygen by absorption in a metallic salt solution, and recovery of an alkene-rich stream from the metallic salt solution. Integrated processes for the production of alkyl carboxylate and alkenyl carboxylate, which processes comprise oxidation of a C 2  to C 4  alkane to produce the corresponding alkene and carboxylic acid, separation of the alkene from a mixture of the alkene, the alkane and oxygen by absorption in a metallic salt solution, and recovery of an alkene-rich stream from the metallic salt solution for use in production of alkyl carboxylate or alkenyl carboxylate.

This application is the U.S. National Phase of International ApplicationPCTIGB03/00686, filed Feb. 12, 2003, which designated the U.S.

The present invention relates to the separation of alkenes from gasmixtures comprising said alkenes, alkanes and oxygen and, in particular,to the separation of ethylene from a mixture of ethylene, ethane andoxygen by absorption in a metallic salt solution.

The present invention also relates to the use of the separation processin (a) hydrocarbon oxidation processes such as the oxidation of a C₂ toC₄ alkane to produce the corresponding alkene and carboxylic acid and(b) in integrated processes in which the alkene and carboxylic acidproduced from a hydrocarbon oxidation process are further used asreactants.

Ethylene and acetic acid may be produced by the catalytic oxidation ofethane. In a typical oxidation process to produce ethylene and aceticacid, ethane, oxygen and optionally ethylene and/or water are introducedinto a reactor. The reactants are contacted with an oxidation catalystsuch as a molybdenum/niobium/vanadium containing catalyst and react toproduce an outlet stream comprising ethylene (either as product orunreacted feed), acetic acid, unreacted ethane and unreacted oxygen. Theoutlet stream is removed from the reactor, condensed and separated intoa gaseous stream and a liquid stream. The gaseous stream comprisingethane, ethylene and oxygen may be further purified to obtain ethylenetherefrom. The liquid stream comprising acetic acid and water may befurther purified.

It is known that the separation of ethylene from hydrocarbons such asethane may be carried out by distillative processes such as cryogenicdistillation and adsorption techniques such as pressure swing adsorptionand reactive adsorption. In addition, where the alkane/alkene gasmixture comprises oxygen, such as the gas mixture produced by theoxydehydrogenation of ethane to ethylene as described, for example, inEP-A-0 262 264, the oxygen is traditionally removed prior to separationof the alkene from the alkane. If the oxygen is not removed prior to theseparation of the hydrocarbons, the separation process can concentratethe oxygen such that the oxygen-containing stream becomes flammable orexplosive.

EP-A-0 943 595 describes a process for separating an alkene such asethylene from a gas mixture comprising the alkene and an alkane such asethane by a pressure swing absorption process comprising the steps ofpassing the gas mixture through a type A zeolite having exchangeablesodium and potassium ions and regenerating the zeolite to produce analkene-enriched gas. Such a pressure swing adsorption system ismechanically complex and a single adsorption cycle gives only a smallenhancement of ethylene concentration. No mention is made of theseparation of alkenes from gas mixtures comprising alkenes, alkanes andoxygen.

WO 00/37399 describes a process for the auto-thermal cracking ofparaffinic hydrocarbons with oxygen in which process the product streamcomprises ethylene, propene, butene and carbon monoxide. Ethylene andpropene are separated from the product stream by contacting the productstream with a solution of a metallic salt capable of selectivelyabsorbing the ethylene and propene and recovering the ethylene and/orpropene from the metallic salt. Prior to treatment with the metallicsalt solution, the product stream is treated to remove components suchas oxygen and carbon dioxide.

The products of the catalytic oxidation of ethane, ethylene and aceticacid, may be reacted in downstream processes to produce alkylcarboxylates such as ethyl acetate or alkenyl carboxylates such as vinylacetate.

In view of the above there remains the need for an alternative and/orimproved process for separating alkenes from a gas mixture comprisingsaid alkenes, alkanes and oxygen.

We have now found that alkene may be separated from a gas mixturecomprising said alkene, alkane and oxygen without the need for priorremoval of the oxygen.

In addition, we have found that the separation of alkene from a gasmixture comprising said alkene, alkane and oxygen may be carried out infewer processing stages than is required by the prior art.

Accordingly the present invention provides a process for separating analkene from a gas mixture comprising said alkene, an alkane and at least0.1 mol % oxygen which process comprises the steps:

-   (a) contacting said gas mixture with a solution of a metallic salt    capable of selectively chemically absorbing the alkene to produce a    chemically absorbed alkene-rich liquid stream;-   (b) recovering the alkene from the metallic salt solution.

Advantageously, the process of the present invention, avoids the needfor costly and energy intensive distillation separation apparatus.

Furthermore, the process of the present invention eliminates or at leastmitigates the need for expensive refrigeration equipment.

More advantageously, the process of the present invention allows thesafe separation of alkene from alkane in the presence of oxygen.

The process of the present invention is particularly useful for theseparation of the alkene from the alkanes where the alkene and alkanebeing separated contain the same number of carbon atoms.

The process of the present invention is especially useful for separatingethylene from gas mixtures containing ethylene, ethane and oxygen.

In the process of the present invention, the alkane is preferably a C₂to C₄ alkane or mixtures thereof such as ethane, propane, butane andmixtures thereof.

Preferably, the alkene is a C₂ to C₄ alkene or mixtures thereof such asethylene, the propenes, the butenes and mixtures thereof.

The concentration of oxygen present in the gas mixture is at least 0.1mol %, such as at least 0.2 mol %. Suitably, the concentration of oxygenin the gas mixture is in the range of 0.1 mol % up to a concentrationwhere the gas mixture is below the flammable range. The oxygenconcentration in the mixture must be such that the alkane-rich productstream is also non-flammable. It will be known to those skilled in theart that the limit of the flammable range is partly dependent on thepressure and temperature of the mixture. The gas mixtures of the processof the present invention should not enter the flammable range at anystage in the process. The gas separation process may be advantageouslyoperated such that a gas mixture is as close as possible to theflammable range whilst remaining non-flammable.

Suitably, the concentration of oxygen in the gas mixture is 0.1 to 10mol %, such as 0.2 to 8 mol %, for example, 0.2 to 6 mol %.

The separation process of present invention is especially applicable toproduct streams of chemical processes. Thus, the process of the presentinvention is particularly useful for separating alkenes from a gasmixture of alkenes, alkanes and oxygen produced in the oxidation of a C₂to C₄ alkane.

Accordingly, the present invention provides a process for the oxidationof a C₂ to C₄ alkane to produce the corresponding alkene and carboxylicacid which process comprises the steps

-   a) contacting in an oxidation reaction zone, said alkane, molecular    oxygen-containing gas, optionally the corresponding alkene and    optionally water, in the presence of at least one catalyst active    for the oxidation of the alkane to the corresponding alkene and    carboxylic acid, to produce a first product stream comprising    alkene, carboxylic acid, alkane, oxygen and water;-   (b) separating in a first separation means at least a portion of the    first product stream into a gaseous stream comprising alkene, alkane    and oxygen and a liquid stream comprising carboxylic acid;-   (c) contacting said gaseous stream with a solution of a metallic    salt capable of selectively chemically absorbing the alkene to    produce a chemically absorbed alkene-rich liquid stream;-   (d) recovering an alkene-rich stream from the metallic salt    solution.

The process of the present invention is also particularly useful whenthe alkene and/or carboxylic acid products of the oxidation process areused at least in part in integrated downstream processes, for example(a) for the production of ester by reacting the carboxylic acid with thealkene or an alcohol or (b) for the production of alkenyl carboxylate bythe reaction of an oxygen-containing gas with the carboxylic acid andalkene. Alkene and/or carboxylic acid may be recovered from the productof the oxidation reaction zone and/or additional alkene and/orcarboxylic acid may be used in the downstream process.

Accordingly, the present invention provides an integrated process forthe production of an alkyl carboxylate which process comprises thesteps:

-   (a) contacting in an oxidation reaction zone, an alkane, molecular    oxygen-containing gas, optionally the corresponding alkene and    optionally water, in the presence of at least one catalyst active    for the oxidation of the alkane to the corresponding alkene and    carboxylic acid, to produce a first non-flammable product stream    comprising alkene, carboxylic acid, alkane, oxygen and water;-   (b) separating in a first separation means at least a portion of the    first product stream into a gaseous stream comprising alkene, alkane    and oxygen and a liquid stream comprising carboxylic acid;-   (c) contacting at least a portion of said gaseous stream with a    solution of a metallic salt capable of selectively chemically    absorbing the alkene to produce a chemically absorbed alkene-rich    liquid stream;-   (d) recovering an alkene-rich stream from the metallic salt solution    and;-   (e) contacting in a second reaction zone at least a portion of said    alkene-rich stream from step (d), and a carboxylic acid, in the    presence of at least one catalyst active for the production of alkyl    carboxylate to produce said alkyl carboxylate,

Also, in another embodiment, the present invention provides anintegrated process for the production of an alkenyl carboxylate whichprocess comprises the steps:

-   (a) contacting in an oxidation reaction zone, an alkane, molecular    oxygen-containing gas, optionally the corresponding alkene and    optionally water, in the presence of at least one catalyst active    for the oxidation of the alkane to the corresponding alkene and    carboxylic acid, to produce a first non-flammable product stream    comprising alkene, carboxylic acid, alkane, oxygen and water;-   (b) separating in a first separation means at least a portion of the    first product stream into a gaseous stream comprising alkene, alkane    and oxygen and a liquid stream comprising carboxylic acid;-   (c) contacting at least a portion of said gaseous stream with a    solution of a metallic salt capable of selectively chemically    absorbing the alkene to produce a chemically absorbed alkene-rich    liquid stream;-   (d) recovering an alkene-rich stream from the metallic salt solution    and;-   (e) contacting in a second reaction zone at least a portion of said    alkene-rich stream obtained in step (d), a carboxylic acid and a    molecular oxygen-containing gas, in the presence of at least one    catalyst active for the production of alkenyl carboxylate to produce    said alkenyl carboxylate.

The separation process of the present invention will now be described inrelation to the oxidation of a C₂ to C₄ alkane to produce a productstream comprising the corresponding alkene, alkane and oxygen andintegrated processes thereof.

In the oxidation reaction, the C₂ to C₄ alkane is preferably ethane, thecorresponding alkene being ethylene and the corresponding carboxylicacid being acetic acid. These products may be reacted in downstreamprocesses to produce ethyl acetate or, with a molecularoxygen-containing gas to produce vinyl acetate.

Typically, the oxidation reaction is performed heterogeneously withsolid catalysts and the reactants in the fluid phase. In this case, theconcentrations of optional alkene and optional water may be controlledas partial pressures in the oxidation reaction zone.

Catalysts active for the oxidation of alkane to alkene and carboxylicacid may comprise any suitable catalysts known in the art, for example,for the oxidation of ethane to ethylene and acetic acid as described inU.S. Pat. No. 4,596,787, EP-A-0407091, DE 19620542, WO 99/20592, DE19630832, WO 98/47850, WO 99/51339, EP-A-0 1043064, WO 9913980, U.S.Pat. No. 5,300,682 and U.S. Pat. No. 5,300,684, the contents of whichare hereby incorporated by reference.

U.S. Pat. No. 4,596,787 relates to a process for the low temperatureoxydehydrogenation of ethane to ethylene using a catalyst having theempirical formula Mo_(a)V_(b)Nb_(c)Sb_(d)X_(e) as therein defined, theelements being present in combination with oxygen.

EP-A-0407091 relates to process and catalyst for the production ofethylene and/or acetic acid by oxidation of ethane and/or ethylene inthe presence of an oxidation catalyst comprising molybdenum, rhenium andtungsten.

DE 19620542 relates to molybdenum, palladium, rhenium based oxidationcatalysts for the production of acetic acid from ethane and/or ethylene.

WO 99/20592 relates to a method of selectively producing acetic acidfrom ethane, ethylene or mixtures thereof and oxygen at high temperaturein the presence of a catalyst having the formula Mo_(a)Pd_(b)X_(c)Y_(d)wherein X represents one or several of Cr, Mn, Nb, Ta, Ti, V, Te and W;Y represents one or several of B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co,Rh, Ir, Cu, Ag, Au, Fe, Ru, Os, K, Rb, Cs, Mg, Ca, Sr, Ba, Nb, Zr, Hf,Ni, P, Pb, Sb, Si, Sn, Tl and U and a=1, b=0.0001 to 0.01, c=0.4 to 1and d=0.005 to 1.

German patent application DE 196 30 832 A1 relates to a similar catalystcomposition in which a=1, b>0, c>0 and d=0 to 2. Preferably, a=1,b=0.0001 to 0.5, c=0.1 to 1.0 and d=0 to 1.0.

WO 98/47850 relates to a process for producing acetic acid from ethane,ethylene or mixtures thereof and a catalyst having the formulaW_(a)X_(b)Y_(c)Z_(d) in which X represents one or several of Pd, Pt, Agand Au, Y represents one or several of V, Nb, Cr, Mn, Fe, Sn, Sb, Cu,Zn, U, Ni, and Bi and Z represents one or several of Li, Na, K, Rb, Cs,Be, Mg, Ca, Sr, Ba, Sc, Y, La, Ti, Zr, Hf, Ru, Os, Co, Rh, Ir, B, Al,Ga, In, Tl, Si, Ge, Pb, P, As and Te, a=1, b>0, c>0 and d is 0 to 2.

WO 99/51339 relates to a catalyst composition for the selectiveoxidation of ethane and/or ethylene to acetic acid which compositioncomprises in combination with oxygen the elementsMo_(a)W_(b)Ag_(c)Ir_(d)X_(e)Y_(f) wherein X is the elements Nb and V; Yis one or more elements selected from the group consisting of Cr, Mn,Ta, Ti, B, Al, Ga, In, Pt, Zn, Cd, Bi, Ce, Co, Rh, Cu, Au, Fe, Ru, Os,K, Rb, Cs, Mg, Ca, Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re andPd; a, b, c, d, e and f represent the gram atom ratios of the elementssuch that 0<a≦1, 0≦b<1 and a+b=1; 0<(c+d)≦0.1; 0<e≦2; and 0≦f≦2.

EP-A-1043064 relates to a catalyst composition for the oxidation ofethane to ethylene and/or acetic acid and/or for the oxidation ofethylene to acetic acid which composition comprises in combination withoxygen the elements molybdenum, vanadium, niobium and gold in theabsence of palladium according to the empirical formula:Mo_(a)W_(b)U_(c)V_(d)Nb_(e)Y_(f) wherein Y is one or more elementsselected from the group consisting of: Cr, Mn, Ta, Ti, B, Al, Ga, In,Pt, Zn, Cd, Bi, Ce, Co, Rh, Ir, Cu, Ag, Fe, Ru, Os, K, Rb, Cs, Mg, Ca,Sr, Ba, Zr, Hf, Ni, P, Pb, Sb, Si, Sn, Tl, U, Re, Te and La; a, b, c, d,e and f represent the gram atom ratios of the elements such that: 0<a≦1;0≦b<1 and a+b=1; 10⁻⁵<c≦0.02; 0<d≦2; 0<e≦1; and 0≦f≦2.

WO 99/13980 relates to a catalyst for the selective oxidation of ethaneto acetic acid of formula: Mo_(a)V_(b)Nb_(c)X_(d) wherein X is at leastone promoter element selected from the group consisting of P, B, Hf; Teand As; a is a number ranging from about 1 to about 5; b is 1; c is anumber ranging from about 0.01 to about 0.5; and d is a number rangingfrom greater than 0 to about 0.1.

U.S. Pat. No. 5,300,682 relates to the use of oxidation catalyst withempirical formula of VP_(a)M_(b)O_(x) where M is one or more of Co, Cu,Re, Fe, Ni, Nb, Cr, W, U, Ta, Ti, Zr, Hf, Mn, Pt, Pd, Sn, Sb, Bi, Ce,As, Ag and Au, a is 0.5 to 3, b is 0 1 and x satisfies the valencerequirements.

U.S. Pat. No. 5,300,684 relates to a fluid bed oxidation reaction usingfor example Mo_(0.37)Re_(0.25)V_(0.26)Nb_(0.07)Sb_(0.03)Ca_(0.02)O_(x).

Other suitable oxidation catalysts for use in the present invention aredescribed in WO 99/13980 which relates to the use of catalysts withelements in combination with oxygen in the relative gram atom ratios ofMO_(a)V_(b)Nb_(c)X_(d) where X=P, B, Hf, Te or As; U.S. Pat. No.6,030,920 which relates to the use of catalysts with elements incombination with oxygen in the relative gram atom ratios ofMo_(a)V_(b)Nb_(c)Pd_(d); WO 00/00284 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a)V_(b)Nb_(c)Pd_(d) and/or Mo_(a)V_(b)La_(c)Pd_(d);U.S. Pat. No. 6,087,297 which relates to the use of catalysts withelements in combination with oxygen in the relative gram atom ratios ofMo_(a)V_(b)Pd_(c)La_(d); WO 00/09260 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a)V_(b)La_(c)Pd_(d)Nb_(e)X_(f) where X=Cu or Cr and eand f can be zero; WO 00/29106 and WO 00/29105 which relate to the useof catalysts with elements in combination with oxygen in the relativegram atom ratios of Mo_(a)V_(b)Ga_(c)Pd_(d) Nb_(e)X_(f) wherein X=La,Te, Ge, Zn, Si, In or W and WO 00/38833 which relates to the use ofcatalysts with elements in combination with oxygen in the relative gramatom ratios of Mo_(a)V_(b)La_(c)Pd_(d)Nb_(e)X_(f) wherein X=Al, Ga, Geor Si, the contents of which are hereby incorporated by reference.

Solid catalysts active for the oxidation of the C₂ to C₄ alkane may besupported or unsupported. Examples of suitable supports include silica,diatomaceous earth, montmorillonite, alumina, silica alumina, zirconia,titania, silicon carbide, activated carbon and mixtures thereof.

Solid catalysts active for the oxidation of the C₂ to C₄ alkane may beused in the form of a fixed or fluidised bed.

The oxidation catalyst would be expected to oxidise at least part of anyalkene fed to the oxidation reaction zone, for example to thecorresponding carboxylic acid.

The molecular oxygen-containing gas used in the oxidation reaction zone,may be air or a gas richer or poorer in molecular oxygen than air. Asuitable gas may be, for example, oxygen diluted with a suitablediluent, for example nitrogen, argon or carbon dioxide. Preferably, themolecular oxygen-containing gas is oxygen. The molecular-oxygencontaining gas may be fed to the oxidation reaction zone as a singlefeed stream comprising the alkane feed. Such an alkane/molecular-oxygengas stream may be obtained from the separation of the alkene fromalkene/alkane/molecular oxygen gas mixture.

Preferably, at least some of the molecular oxygen-containing gas is fedto the oxidation reaction zone independently from the alkane andoptional alkene feeds, and any recycle streams.

Suitably, the concentration of the molecular-oxygen containing gas (asfresh feed and/or recycle) is such that the concentration of oxygen isfrom greater than 0 and up to and including 20 mol % of the total feed,including recycles, to the oxidation reaction zone, preferably 2-15 mol%.

The alkane and alkene fed into the oxidation reaction zone may besubstantially pure or may be admixed, for example, with one or more ofnitrogen, argon, methane, carbon dioxide, carbon monoxide, hydrogen, andlow levels of other C₂ to C₄ alkenes/alkanes.

Suitably, the concentration of alkene (as fresh feed and/or recyclecomponent) is from 0 and up to and including 50 mol % of the total feed,including recycles, to the oxidation reaction zone, preferably from 1 to20 mol %, more preferably from 1 to 15 mol %.

Suitably, the concentration of water (as fresh feed and/or recyclecomponent) is from 0 to 50 mol % inclusive of the total feed, includingrecycles, to the oxidation reaction zone, preferably from 0 to 25 mol %.

In one embodiment of the present invention, the alkene, such asethylene, and water are co-fed into the oxidation reaction zone.

Suitably, the alkene, for example, ethylene, and water may be used in aratio of 1:0.1-250 by weight, such as 1:0.1-100 or 1:0.1-50 butpreferably in a ratio 1:0.1-10 by weight.

When solid catalysts are used in the oxidation reaction zone, thealkane, corresponding alkene, molecular-oxygen containing gas, optionalwater and any recycle gases are preferably passed through the oxidationreaction zone with a residence time corresponding to a combined gashourly space velocity (GHSV) of 500-10,000 hr⁻¹; the GHSV being definedas volume (calculated at STP) of gas passing through the reactor dividedby the bulk volume of settled catalyst.

The oxidation reaction may suitably be carried out at a temperature inthe range from 100 to 400° C., typically in the range 140 to 350° C.

The oxidation reaction may suitably be carried out at atmospheric orsuperatmospheric pressure, for example, in the range from 5 to 27 barg.

Typically, alkane conversions in the range 1 to 99% may be achieved inthe oxidation reaction of the present invention.

Typically, oxygen conversions in the range 30 to 99.99% may be achievedin the oxidation reaction of the present invention.

The concentration of oxygen in the product stream will depend to someextent on the degree of alkane conversion and the degree of selectivityto products. A high alkane conversion will result in a low concentrationof oxygen present in the product stream. A high selectivity to productalkene will result in a high concentration of oxygen in the productstream.

The maximum (safe) concentration of oxygen in the product stream isdetermined by the flammable range of the oxygen to alkane ratio afterseparation of the alkene therefrom.

Thus, although the concentration of oxygen present in the product streamfrom the oxidation reaction zone may be less than 0.1 mol % it istypically at least 0.1 mol %, such as at least 0.2 mol %. Suitably,provided the product stream is non-flammable, the concentration ofoxygen in the product stream is in the range of 0.1 mol % up to andincluding 10 mol %, such as 0.2 to 8 mol %, for example, 0.2 to 6 mol %.

In the oxidation reaction, the catalyst suitably has a productivity inthe range 10 to 10000 grams of carboxylic acid, such as acetic acid, perhour per kilogram of catalyst.

In the oxidation reaction, the catalyst suitably has a productivity inthe range 5 to 5000 grams of alkene, such as ethylene, per hour perkilogram of catalyst.

Carbon monoxide can have an adverse effect on some catalysts used in theproduction of vinyl acetate. Thus, depending on the nature of thecatalyst employed, it is desirable that the first product stream shouldhave a low concentration of carbon monoxide by-product.

Thus, it is also preferred to use a catalyst in the oxidation reactionzone that gives negligible carbon monoxide by-product. An additionalcatalyst component in the oxidation reaction zone may be used to oxidisecarbon monoxide to carbon dioxide. The additional catalyst component maybe present in the oxidation catalyst or catalysts or in a secondaryreaction zone or may be present as a separate catalyst in the oxidationreaction zone.

When ethane is used as a reactant for the oxidation reaction, theproduct stream comprises acetic acid, ethylene, unreacted ethane, oxygenand water and may also contain inert gas components such as argon andnitrogen and the by-products, acetaldehyde, carbon monoxide and carbondioxide. Acetaldehyde and carbon monoxide may be converted by themolecular oxygen-containing gas to produce acetic acid and carbondioxide respectively, either in downstream processes or, afterrecycling, in the oxidation reaction zone.

Ethylene is present in the product stream of the oxidation reaction asunconverted reactant ethylene from the feed and/or as oxidation productof the ethane reactant.

The product stream from the oxidation reaction zone is separated in afirst separation means into a gaseous stream comprising the alkene,unreacted alkane and oxygen and a liquid stream comprising thecarboxylic acid. Any suitable separation means known in the art may beemployed such as a membrane separation unit, condensing unit or adistillation unit. Preferably, the separation means employed is acondenser.

Where the product stream from the oxidation reaction comprises aceticacid, ethylene, ethane, oxygen and water, the product stream may be, andis preferably, separated by condensation into an overhead gaseous streamcomprising ethylene, ethane and oxygen and a base liquid streamcomprising acetic acid and water. In general, the gaseous stream willalso comprise carbon oxides such as carbon dioxide.

Optionally, carboxylic acid and/or alkene may be recovered from theproduct stream of the oxidation reaction.

The gaseous stream from the first separation means is contacted with asolution of a metal salt capable of selectively chemically absorbing thealkene to produce a chemically absorbed alkene-rich liquid stream.

Suitable metallic salts are those capable of forming a complex with thealkene.

Where, the alkene is ethylene, suitable metal salts comprise, chromium,copper (I), manganese, nickel, iron, mercury, silver, gold, platinum,palladium, rhodium, ruthenium, osmium, molybdenum, tungsten and rhenium.

Preferably, the metal salt comprises silver or copper (I), mostpreferably silver.

Where the metal salt is a silver salt, the silver salt is preferably,silver nitrate or silver fluoroborate.

Where the metal salt is a copper (I) salt, the copper (I) salt ispreferably copper (I) acetate, copper (I) nitrate or copper (I)sulphate, most preferably copper (I) nitrate.

The metal solution may be aqueous or may comprise an organicnitrogen-containing compound such as pyridine, piperidine,hydroxy-propionitrile, diethylenetriamine, acetonitrile, formamide,acetamide and derivatives thereof.

Preferably, the metallic salt solution is an aqueous solution.

Where the metal salt is copper (I), the concentration of metal salt tonitrogen-containing compound is suitably in the range 1:1 to 1:6,preferably, 1:2.

The concentration of metal salt in the solution is suitably at least 0.5moles of metal salt per litre of solvent, preferably, at least 2 molesof metal salt per litre of solvent.

Neither the alkane nor the oxygen present in the gaseous stream forms acomplex with the metallic salt solution to any significant extent.

The contacting of the gaseous stream with the metallic salt solution maybe carried out in any suitable means such as in an absorber column. Theabsorber column may be fitted with trays or packing such as raschigrings or structured packing. Preferably, the absorber column is fittedwith packing.

To improve the purity of the alkene, the absorber column is suitablyequipped with a reboiler.

Preferably, the absorber column is operated with counter-current flow ofgas and metallic salt solution.

Suitably, the contacting may be carried out at a temperature in therange from −10 to 300° C., preferably, 0 to 100° C.

Suitably, the contacting may be carried out at a pressure in the rangefrom 1 to 70 barg, preferably, 3 to 30 barg.

Where the contacting is carried out in an absorber column, the metallicsalt solution comprising the metal salt/alkene complex may be removedfrom the base of the absorber.

As the alkane and oxygen do not complex to any significant extent withthe metallic salt solution they are removed as an overhead stream fromthe absorber column.

Trace amounts of oxygen and/or alkane absorbed in the metallic saltsolution are mostly removed from the solution with the alkene.

An alkene-rich stream may be recovered from the metallic salt solutionby heat, reduced pressure or by a combination thereof. Preferably, thesolution is subjected to a reduced pressure such that the complexdecomposes to release the alkene.

The pressure used for recovery of the alkene-rich stream from themetallic salt solution may be 2 to 98% of the absolute pressure used toform the metal salt/alkene complex, preferably 10 to 80% of the absolutepressure used to form the complex.

Alternatively, the alkene-rich stream may be recovered from the metallicsalt solution by degassing at a temperature in the range from 0 to 80°C., preferably in the range 15 to 35° C. above the temperature offormation of the complex.

The alkene-rich stream may also be recovered from the solution using acombination of reduced pressure and increased temperature.

The pressure reduction may be carried out in one or more stages, forexample, in one or more flashing apparatus.

Where one or more flashing apparatus are employed, the alkene-richstream is removed therefrom as an overhead stream. The overhead streammay be compressed prior to be being optionally dried. Alternatively, theoverhead stream may be dried prior to being compressed. Where thealkene-rich stream is compressed, it may be compressed to a pressuresuitable for feeding to the second reaction zone. Suitably, it may becompressed to the pressure of any additional alkene feed to the secondreaction zone.

The alkene-free complex may be recycled for re-use in the absorber.

The alkene-rich stream will comprise the alkene and may comprise lowlevels of alkane and oxygen and other impurities such as carbon dioxide.

Suitably, the alkene-rich stream, such as an ethylene-rich stream,comprises at least 50% alkene, such as at least 80% alkene. Preferablythe alkene-rich stream comprises at least 90% alkene, more preferably,95% alkene, and most preferably, at least 99% alkene.

The alkene-rich stream may be recovered from the metallic salt solutionin one or more absorption/desorption stages, such as one absorption andtwo desorption stages.

Advantageously, the use of an alkene feed to the second reaction zonehaving reduced levels of impurities allows the amount of purge gas whichhas to be vented from the second reaction zone to be reduced and hencethe loss of alkene from the second reaction zone is also reduced.

The alkane and oxygen stream (alkane-rich stream) may comprise lowlevels of alkene and other impurities such as carbon dioxide. Thealkane-rich stream must be non-flammable. The flammable range willdepend on, for example, temperature and pressure of the alkane-richstream, however, typically, the oxygen concentration in the alkane-richstream may be in the range 0.1 to 10 mol %.

In a preferred embodiment of the process of the present invention, thealkene/alkane/oxygen gaseous stream (gaseous stream from the firstseparation means), prior to being contacted with the metallic saltsolution, is treated to remove components such as carbon dioxide, andoxygenates such as acetaldehyde.

The alkane and oxygen containing gas stream may be fed as one or morestreams to the oxidation reaction zone together with additional alkane.

Optionally, prior to being fed into the oxidation reaction zone, thealkane and oxygen containing stream may be separated into separatealkane and oxygen gas streams.

The additional alkane may be fresh alkane and/or may be unreacted alkanefrom the oxidation reaction zone which has been recycled after the firstseparation means to the oxidation reaction zone.

The alkane/oxygen stream and additional alkane may be introduced intothe oxidation reaction zone either as separate feed streams or as asingle feed stream comprising both the alkane/oxygen and additionalalkane.

The alkene-rich stream is fed as one or more streams, to a secondreaction zone together with additional molecular oxygen-containing gas,optional additional alkene and carboxylic acid to produce alkenylcarboxylate, such as vinyl acetate.

The alkene-rich stream and additional alkene may be introduced into thesecond reaction zone either as separate feed streams or as a single feedstream comprising both alkene-rich stream and additional alkene.

The additional alkene may be fresh alkene and/or recycled alkene fromthe second reaction zone and/or a portion of the alkane/alkene streamfrom the oxidation reaction zone.

Additional alkene introduced into the second reaction zone for theproduction of alkenyl carboxylate may be substantially pure or may beadmixed, for example, with one or more of nitrogen, argon, methane,carbon dioxide, carbon monoxide, hydrogen, and low levels of other C₂ toC₄ alkenes/alkanes.

Suitably, the concentration of alkene (optional additional alkene feedand alkene-rich stream feed), such as ethylene, fed to the secondreaction zone is at least 50 mol % of the total feed to the secondreaction zone, preferably, at least 55 mol %, more preferably at least60 mol %. Suitably, the concentration of alkene is up to 85 mol % of thetotal feed to the second reaction zone, preferably, in the range atleast 50 mol % to 80 mol %, such as at least 55 mol % to 80 mol %.

Catalysts known in the art for the production of alkenyl carboxylatesmay be used in the process of the present invention. Thus, catalystactive for the production of vinyl acetate which may be used in a secondreaction zone of the present invention may comprise, for example,catalysts as described in GB 1 559 540; U.S. Pat. No. 5,185,308 andEP-A-0672453 the contents of which are hereby incorporated by reference.

GB 1 559 540 describes a catalyst active for the preparation of vinylacetate by the reaction of ethylene, acetic acid and oxygen, thecatalyst consisting essentially of: (1) a catalyst support having aparticle diameter of from 3 to 7 mm and a pore volume of from 0.2 to 1.5ml/g, a 10% by weight water suspension of the catalyst support having apH from 3.0 to 9.0, (2) a palladium-gold alloy distributed in a surfacelayer of the catalyst support, the surface layer extending less than 0.5mm from the surface of the support, the palladium in the alloy beingpresent in an amount of from 1.5 to 5.0 grams per litre of catalyst, andthe gold being present in an amount of from 0.5 to 2.25 grams per litreof catalyst, and (3) from 5 to 60 grams per litre of catalyst of alkalimetal acetate.

U.S. Pat. No. 5,185,308 describes a shell impregnated catalyst activefor the production of vinyl acetate from ethylene, acetic acid and anoxygen containing gas, the catalyst consisting essentially of: (1) acatalyst support having a particle diameter from about 3 to about 7 mmand a pore volume of 0.2 to 1.5 ml per gram, (2) palladium and golddistributed in the outermost 1.0 mm thick layer of the catalyst supportparticles, and (3) from about 3.5 to about 9.5% by weight of potassiumacetate wherein the gold to palladium weight ratio in said catalyst isin the range 0.6 to 1.25.

EP-A-0672453 describes palladium containing catalysts and theirpreparation for fluid bed vinyl acetate processes.

Typically, the production of alkenyl carboxylate such as vinyl acetatein the second reaction zone is carried out heterogeneously with thereactants being present in the gas phase.

The molecular oxygen-containing gas used in the second reaction zone forthe production of alkenyl carboxylate may comprise unreacted molecularoxygen-containing gas from step (a) and/or additional molecularoxygen-containing gas.

The additional molecular oxygen-containing gas, if used, may be air or agas richer or poorer in molecular oxygen than air. A suitable additionalmolecular oxygen-containing gas may be, for example, oxygen diluted witha suitable diluent, for example nitrogen, argon or carbon dioxide.Preferably, the additional molecular oxygen-containing gas is oxygen.Preferably, at least some of the molecular oxygen-containing gas is fedindependently to the second reaction zone from the alkene and carboxylicacid reactants.

The carboxylic acid fed to the second reaction zone for the productionof alkenyl carboxylate may comprise fresh and/or recycle acid.Preferably, at least a portion of the carboxylic acid introduced in tothe second reaction zone comprises carboxylic acid produced from theoxidation reaction zone.

The fresh and recycle carboxylic acid may be introduced into the secondreaction zone either as separate feed streams or as a single feed streamcomprising both fresh and recycle acid.

The carboxylic acid fed to the second reaction zone for the productionof alkenyl carboxylate may comprise at least a portion of the acidobtained from downstream processes such as from the separation of theacid from a mixture of the acid/alkenyl carboxylate/water.

At least part of the carboxylic acid fed to the second reaction zone maybe liquid.

When solid catalysts are used in the second reaction zone for theproduction of alkenyl carboxylate, the alkene from the second separationmeans, the carboxylic acid from the oxidation reaction zone, anyadditional alkene or carboxylic acid reactants, any recycle streams andmolecular oxygen-containing gas are preferably passed through the secondreaction zone at a combined gas hourly space velocity (GHSV) of500-10,000 hr⁻¹.

The second reaction zone for the production of alkenyl carboxylate maysuitably be operated at a temperature in the range from 140 to 200° C.

The second reaction zone for the production of alkenyl carboxylate maysuitably be operated at a pressure in the range 50 to 300 psig.

The second reaction zone for the production of alkenyl carboxylate maysuitably be operated as either a fixed or a fluidised bed process.

Carboxylic acid conversions in the range 5 to 80% may be achieved in thesecond reaction zone for the production of alkenyl carboxylate.

Oxygen conversions in the range 20 to 100% may be achieved in the secondreaction zone for the production of alkenyl carboxylate.

Alkene conversions in the range 3 to 100% may be achieved in the secondreaction zone for the production of alkenyl carboxylate.

In the second reaction zone for the production of alkenyl carboxylate,the catalyst suitably has a productivity in the range 10 to 10000 gramsof alkenyl carboxylate per hour per kg of catalyst.

When the alkane used in the process of the present invention is ethane,the product stream from the second reaction zone for the production ofalkenyl carboxylate may comprise vinyl acetate, water and acetic acidand optionally also unreacted ethylene, ethane, oxygen, acetaldehyde,nitrogen, argon, carbon monoxide and carbon dioxide. Such a productstream may be separated by azeotropic distillation into an overheadfraction comprising vinyl acetate and water and a base fractioncomprising acetic acid and water. The base fraction is be removed fromthe distillation column as liquid from the bottom of the column. Inaddition, a vapour from one or more stages above the bottom of thecolumn may also be removed. Prior to such a distillation step, ethylene,ethane, acetaldehyde, carbon monoxide and carbon dioxide, if any, may beremoved from the second product stream, suitably as an overhead gaseousfraction from a scrubbing column, in which a liquid fraction comprisingvinyl acetate, water and acetic acid is removed from the base. Theethylene and/or ethane may be recycled to the oxidation reaction zoneand/or the second reaction zone and/or the second separation means.

The alkenyl carboxylate, for example, vinyl acetate is recovered fromthe overhead fraction, suitably for example by decantation. Therecovered alkenyl carboxylate, such as vinyl acetate, may, if desired,be further purified in known manner.

The base fraction comprising carboxylic acid, such as acetic acid andwater may be recycled, with or preferably without further purification,to the second reaction zone. Alternatively, the carboxylic acid isrecovered from the base fraction and may be further purified if desired,in known manner, for example by distillation.

The invention will now be illustrated by reference to the FIGURE.

The FIGURE represents in schematic block-diagram, apparatus suitable foruse in a process of the present invention.

The apparatus comprises an oxidation reaction zone (1) provided with asupply of ethane and optionally ethylene (3), a supply of a molecularoxygen-containing gas (4), a supply of recycle gas comprising ethane andethylene (5), a supply (19) of ethane and oxygen from anethylene/ethane/oxygen absorber column (21), and an outlet (18) for afirst product stream. Depending on the scale of the process, theoxidation reaction zone (1) may comprise either a single reactor orseveral reactors in parallel or series.

The apparatus also comprises a scrubber (6) for separating the firstproduct stream into a gaseous stream comprising ethylene, ethane andcarbon oxides and a liquid stream comprising acetic acid and water.Optionally, the apparatus comprises means (not shown) for removing waterfrom the acetic acid, such as a distillation unit.

The apparatus also comprises a series of flashing apparatus (flashingvalves and drums) (22,23) for subjecting the ethylene/metallic saltcomplex obtained as a base fraction from absorber column (21) to areduced pressure and an optional compressor (24) for compressing anethylene-rich overhead stream from the flashing apparatus (22, 23).

The apparatus also comprises a second reaction zone (2) foracetoxylation of ethylene to vinyl acetate which is provided with means(17) for conveying at least a portion of the acetic acid from thescrubber (6) into the second reaction zone, optionally via a means forremoving water from the liquid stream, a supply of molecularoxygen-containing gas (9), a supply of recycle acetic acid (10), anoptional supply or supplies of acetic acid and/or ethylene (8) and asupply (25) of ethylene from the optional compressor (24). Depending onthe scale of the process, the second reaction zone (2) may compriseeither a single reactor or several reactors in parallel or in series.

The apparatus further comprises a scrubber (12) for the product from thesecond reaction zone; means (13) for separating acetic acid from theproduct of the second reaction zone; vinyl acetate purfication means(14); optional acetic acid purification means (15) and one or moreseparation means (16) for separating carbon dioxide from the gaseousstream obtained from scrubber (6) and optionally for recovery ofethylene product.

In use, the oxidation reaction zone (1) is provided with at least onecatalyst each active for the oxidation of the ethane to form acetic acidand ethylene. Suitably the oxidation catalysts are solid catalysts.Molecular oxygen-containing gas is fed to the oxidation reaction zone(1) from supply (4) through one or more inlets. A gaseous feedstockcomprising ethane and ethylene is fed to the oxidation reaction zone (1)from supply (3). Recycle gas comprising ethane and ethylene is also fedto the oxidation reaction zone (1) from supply (5). Ethane and oxygenfrom the absorber column (21) is fed to the oxidation reaction zone (1)from supply (19)

The molecular oxygen-containing gas, ethane, ethylene and recycle gasare introduced into the oxidation reaction zone (1) through one or moreinlets separately or in partial or complete combination. Optionally atleast one of the streams fed to the oxidation reactor also compriseswater.

In the oxidation reactor a first product stream is produced whichcomprises ethylene (as product and/or unreacted feed), acetic acid,water, optionally unconsumed molecular oxygen-containing gas, unreactedethane and by-products such as carbon monoxide, carbon dioxide, inertsand acetaldehyde. At least a portion of this product stream is passed toa scrubber (6) from which a gaseous stream comprising ethylene, ethane,oxygen and the carbon oxides and a liquid stream comprising acetic acidand water are removed. At least a portion of the gaseous stream is fed,after separating by-products such as carbon dioxide in separation means(16) and optionally recovering a portion of the ethylene product bymethods known in the art, to a high pressure absorber column (21). Atleast a portion of a gaseous stream comprising ethylene and ethane fromseparation means (16) is recycled to the oxidation reaction zone (1) viasupply (5). The gaseous stream comprising ethylene, ethane and oxygen isfed to the absorber column (21) which contains silver nitrate solutionwith which the ethylene reacts to form a silver nitrate/ethylenecomplex. The ethane and oxygen are not complexed and are removed as anoverhead stream from the column. A solution containing the silvernitrate/ethylene complex is removed from the base of the absorbercolumn. The solution is passed to a series of flash-drums (22, 23) whereit is subjected to a reduced pressure. Under such conditions, the silvernitrate/ethylene complex decomposes releasing ethylene. Ethylene isrecovered as an overhead stream. The overhead ethylene stream is fed toa compressor (24) prior to being fed via supply (25) to the secondreaction zone (2). The ethane/oxygen stream from the absorber column isfed to the oxidation reaction zone (1) via supply (19)

Acetic acid may be recovered from the liquid stream of scrubber (6), forexample by distillation.

At least a portion of the acetic acid from the liquid stream is fed bymeans (17), optionally via a water removal means (not shown), into thesecond reaction zone (2), which is provided with an acetoxylationcatalyst, suitably a solid catalyst. A molecular oxygen-containing gasis fed to the second reaction zone from supply (9). Acetic acid is fedto the second reaction zone from recycle supply (10). Optionally,additional ethylene and/or acetic acid may be fed to the second reactionzone from supply or supplies (8). Ethylene is fed from the separationmeans (21) to the second reaction zone from supply (22). Acetic acidfrom the liquid scrubber stream, molecular oxygen-containing gas,recycle acetic acid, optional additional supplies of ethylene and/oracetic acid, and ethylene from the separation means (21) are fed intothe second reaction zone through one or more inlets separately or inpartial or complete combination.

In the second reaction zone the ethylene, acetic acid and molecularoxygen react to produce a second product stream comprising vinylacetate.

The second reaction product is passed to scrubber (12) from which gasand liquid are separated. Carbon dioxide is separated from the gas andoptionally ethylene product recovered, in one or more separation stages(not shown) by methods known in the art. The remaining ethylene andethane may be recycled to the first and/or second reaction zones. Aceticacid is separated in separation means (13) from the scrubber liquid andis recycled to the second reaction zone via recycle supply (10).Optionally, acetic acid product may be recovered from the recycle streamby means (15), for example by distillation. Vinyl acetate product isrecovered from the scrubber liquid by means (14), for example bydistillation.

1. A process for the oxidation of a C₂ to C₄ alkane to produce thecorresponding alkene and carboxylic acid which process comprises thesteps: (a) contacting in an oxidation reaction zone, an alkane,molecular oxygen-containing gas, optionally the corresponding alkene andoptionally water, in the presence of at least one catalyst active forthe oxidation of the alkane to the corresponding alkene and carboxylicacid, to produce a first product stream comprising alkene, carboxylicacid, alkane, oxygen and water; (b) separating in a first separationmeans at least a portion of the first product stream into a gaseousstream comprising alkene, alkane and oxygen and a liquid streamcomprising carboxylic acid; (c) contacting said gaseous stream with asolution of a metallic salt capable of selectively chemically absorbingthe alkene to produce a chemically absorbed alkene-rich liquid stream;and (d) recovering an alkene-rich stream from the metallic saltsolution.
 2. An integrated process for the production of an alkylcarboxylate which process comprises the steps: (a) contacting in anoxidation reaction zone, an alkane, molecular oxygen-containing gas,optionally the corresponding alkene and optionally water, in thepresence of at least one catalyst active for the oxidation of the alkaneto the corresponding alkene and carboxylic acid, to produce a firstnon-flammable product stream comprising alkene, carboxylic acid, alkane,oxygen and water; (b) separating in a first separation means at least aportion of the first product stream into a gaseous stream comprisingalkene, alkane and oxygen and a liquid stream comprising carboxylicacid; (c) contacting at least a portion of said gaseous stream with asolution of a metallic salt capable of selectively chemically absorbingthe alkene to produce a chemically absorbed alkene-rich liquid stream;(d) recovering an alkene-rich stream from the metallic salt solution;and (e) contacting in a second reaction zone at least a portion of saidalkene-rich stream from step (d), and a carboxylic acid, in the presenceof at least one catalyst active for the production of alkyl carboxylateto produce said alkyl carboxylate.
 3. An integrated process for theproduction of an alkenyl carboxylate which process comprises the steps:(a) contacting in an oxidation reaction zone, an alkane, molecularoxygen-containing gas, optionally the corresponding alkene andoptionally water, in the presence of at least one catalyst active forthe oxidation of the alkane to the corresponding alkene and carboxylicacid, to produce a first non-flammable product stream comprising alkene,carboxylic acid, alkane, oxygen and water; (b) separating in a firstseparation means at least a portion of the first product stream into agaseous stream comprising alkene, alkane and oxygen and a liquid streamcomprising carboxylic acid; (c) contacting at least a portion of saidgaseous stream with a solution of a metallic salt capable of selectivelychemically absorbing the alkene to produce a chemically absorbedalkene-rich liquid stream; (d) recovering an alkene-rich stream from themetallic salt solution; and (e) contacting in a second reaction zone atleast a portion of said alkene-rich stream obtained in step (d), acarboxylic acid and a molecular oxygen-containing gas, in the presenceof at least one catalyst active for the production of alkenylcarboxylate to produce said alkenyl carboxylate.
 4. The processaccording to claim 3, wherein, in step (e), said alkene-rich stream isfed to the second reaction zone as one or more streams, together withoptional additional alkene.
 5. The process according to claim 4, whereinthe additional alkene may be fresh alkene and/or recycled alkene fromthe second reaction zone and/or a portion of the alkane/alkene streamfrom the oxidation reaction zone.
 6. The process according to claim 3,wherein the concentration of alkene (optional additional alkene feed andalkene-rich stream feed) fed to the second reaction zone is at least 50mol % of the total feed to the second reaction zone.
 7. The processaccording to claim 6 wherein the concentration of alkene is at least 60mol % of the total feed to the second reaction zone.
 8. The processaccording to claim 6 wherein the concentration of alkene is up to 85 mol% of the total feed to the second reaction zone.
 9. The processaccording to claim 3 wherein the molecular oxygen-containing gas used inthe second reaction zone for the production of alkenyl carboxylatecomprises unreacted molecular oxygen-containing gas from step (a) and/oradditional molecular oxygen-containing gas.
 10. The process according toclaim 9, wherein the additional molecular oxygen-containing gas isoxygen.
 11. The process according to claim 3 wherein at least some ofthe molecular oxygen-containing gas is fed independently to the secondreaction zone from the alkene and carboxylic acid reactants.
 12. Theprocess according to claim 3 wherein the carboxylic acid introduced into the second reaction zone comprises carboxylic acid produced from theoxidation reaction zone.
 13. A process according to claim 1 wherein thealkane is selected from the group consisting of C₂ to C₄ alkanes andmixtures thereof.
 14. A process according to claim 1 wherein the alkaneis ethane, the corresponding alkene is ethylene and the correspondingcarboxylic acid is acetic acid.
 15. A process according to claim 1wherein the molecular oxygen-containing gas in step (a) is oxygen. 16.The process according to claim 1 wherein the concentration of themolecular oxygen-containing gas (as fresh feed and/or recycle) is fromgreater than 0 to 20 mol % of the total feed, including recycles, to theoxidation reaction zone.
 17. The process according to claim 1 whereinthe concentration of alkene (as fresh feed and/or recycle component) isfrom 0 to 50 mol % of the total feed, including recycles, to theoxidation reaction zone.
 18. The process according to claim 17, whereinthe concentration of alkene is from 1 to 20 mol % of the total feed tothe oxidation reaction zone.
 19. The process according to claim 1wherein the concentration of water (as fresh feed and/or recyclecomponent) is from 0 to 50 mol % of the total feed, including recycles,to the oxidation reaction zone.
 20. The process according to claim 19,wherein the concentration of water is from 0 to 25 mol % of the totalfeed to the oxidation reaction zone.
 21. The process according to claim1 wherein the alkene and water are co-fed into the oxidation reactionzone.
 22. The process according to claim 1 wherein the alkene and waterare used in a ratio of 1:0.1-250 by weight.
 23. A process according toclaim 1 wherein the concentration of oxygen present in the gaseousstream from the first separation means is at least 0.1 mol %.
 24. Aprocess according to claim 23, wherein the concentration of oxygenpresent in the gaseous stream from the first separation means is atleast 0.2 mol %.
 25. A process according to claim 24, wherein theconcentration of oxygen present in the gaseous stream from the firstseparation means is 0.1 to 10 mol %.
 26. A process according to claim 1wherein the first separation means is a membrane separation unit,condensing unit or a distillation unit.
 27. A process according to claim26, wherein the separation means employed is a condenser.
 28. Theprocess according to claim 1 wherein the alkene is ethylene and themetal salt capable of selectively chemically absorbing the alkenecomprises chromium, copper (I), manganese, nickel, iron, mercury,silver, gold, platinum, palladium, rhodium, ruthenium, osmium,molybdenum, tungsten or rhenium.
 29. The process according to claim 28,wherein the metallic salt comprises silver or copper (I).
 30. Theprocess according to claim 29, wherein the metallic salt is a silversalt.
 31. The process according to claim 30, wherein the silver salt issilver nitrate or silver fluoroborate.
 32. The process according toclaim 29, wherein the metallic salt is copper (I) acetate, copper (I)nitrate or copper (I) sulphate.
 33. The process according to claim 1wherein the metal solution is aqueous or comprises an organicnitrogen-containing compound.
 34. The process according to claim 1wherein the contacting of the gaseous stream from the first separationmeans with the metallic salt solution is carried out in an absorbercolumn.
 35. The process according to claim 34, wherein the metallic saltsolution comprising the metal salt/alkene complex is removed from thebase of the absorber column, and alkane and oxygen are removed as anoverhead stream from the absorber column.
 36. The process according toclaim 35, wherein the alkane and oxygen containing gas stream is fed asone or more streams to the oxidation reaction zone together withadditional alkane.
 37. The process according to claim 36, wherein, priorto being fed to the oxidation reaction zone, the alkane and oxygencontaining stream is separated into separate alkane and oxygen gasstreams.
 38. The process according to claim 36, wherein the additionalalkane is fresh alkane and/or unreacted alkane from the oxidationreaction zone which has been recycled after the first separation meansto the oxidation reaction zone.
 39. The process according to claim 36wherein the alkane/oxygen stream and the additional alkane areintroduced into the oxidation reaction zone together either as separatefeed streams or as a single feed stream comprising both thealkane/oxygen and the additional alkane.
 40. The process according toclaim 1 wherein the alkene-rich stream is recovered from the metallicsalt solution complex by heat, reduced pressure or by a combinationthereof.
 41. The process according to claim 40, wherein the solution issubjected to a reduced pressure such that the complex decomposes torelease the alkene.
 42. The process according to claim 1 wherein thealkene-rich stream comprises at least 50% alkene.
 43. The processaccording to claim 42, wherein the alkene-rich stream comprises at least90% alkene.
 44. The process according to claim 1 wherein the gaseousstream from the first separation means, prior to being contacted withthe metallic salt solution, is treated to remove components selectedfrom the group consisting of carbon dioxide and oxygenates.
 45. Artintegrated process for the production of vinyl acetate which processcomprises the steps: (a) contacting in an oxidation reaction zone,ethane, molecular oxygen-containing gas, optionally ethylene andoptionally water, in the presence of at least one catalyst active forthe oxidation of ethane to ethylene and acetic acid, to produce a firstnon-flammable product stream comprising ethylene, acetic acid, ethane,oxygen and water; (b) separating in a first separation means at least aportion of the first product stream into a gaseous stream comprisingethylene, ethane and oxygen and a liquid stream comprising acetic acid;(c) contacting at least a portion of said gaseous stream with a solutionor a metallic salt capable of selectively chemically absorbing ethyleneto produce a chemically absorbed ethylene-rich liquid stream; (d)recovering an ethylene-rich stream from the metallic salt solution; and(e) contacting in a second reaction zone at least a portion of saidethylene-rich stream obtained in step (d), acetic acid and a molecularoxygen-containing gas, in the presence of at least one catalyst activefor the production of vinyl acetate to produce vinyl acetate.